pure class A?

All "single ended" amplifiers are by definition Class A,
if they are high-fidelity amplifiers.

All that single ended means is that the amplifying element
(tube or transistor) is alone, and not in push-pull. So,
this means that it is starting out, when doing *nothing*
at all "halfway" on. In other words, biased Class A.

By being halfway on, it can go all the way ON,and all the
way off, this being the AC signal that it can produce. If you are sitting there thinking, 'hey, if it's on halfway all the time, that's wasting a lot of power' then you've got it right. Class A single ended amps tend to waste about 75% of
their power as heat, not audio output.

If there is an advantage to Class A amps of any type it is that at low levels the inherent linearity of the output stage is better than a class AB or class B which has some non-linearity happening as one set of output devices switches all the way *off* as the signal (AC) swings through "zero" going from + to - and back to + again (through zero twice there).

The linearity issue is small, as is the deviation from linear gain - you can think of it as a dip or peak in the frequency response of a crossover, not summing quite right. But of course, this is not frequency dependent. It is however usually *bias* dependent. Since by definition all distortion is deviation from ideal gain, non-linearity in the "crossover" region is measured as distortion.

Doug Self has an excellent discussion of this on his website and in his book, although I may not agree with all of his conclusions on the subject, his technical work is impeccable. He has a "blameless" Class B design that measures better than most Class AB designs...

But that's not single ended. Single-ended is what is in most
tube preamps, the classic cascade circuit. A series of simple class A stages one run into the next.

Single ended class A tubes require a transformer in place of the resistor used in preamps at the plate, or else a plate resistor or choke and a cap blocking the DC to a transformer (a popular idea these days). In any event the output is created by sinking current through the tube, causing a voltage drop in the resistor, or through the transformer (more or less).

Push-pull is in effect replacing the resistor with an active device that amplifies the signal but *inverted* in polarity - so that when one side is ON, the other is OFF.

Two guys on opposite ends of a big old saw - one pushes, the other pulls (in phase output, out of phase input)! That's
push-pull.

Single ended - one guy on one side of the saw - pushes and
pulls all the time.

Single ended, parallel- two guys on the same side of the saw.

Push-pull class B - One guy pulls, then the other guy pulls - or you can think of it as one guy pushes and pulls *exactly* 1/2 way,
then the other guy completes the cycle, pulling and pushing back 1/2 way (a more accurate description...).

Class AB - as with class B, but in the middle, their efforts
overlap a little bit.

Class H? guys with jack hammers, pound the damn thing from one side to the other with monster pulses... or is that class G... I forget.

_-_-bear

Sorry i missed your questions folks. Luckily, Bear stepped in and helped us out. He is far more capable of explaining most technical things than i am. Sean
>

Bear, great job! Thanks. Sean don't underestimate your self.

Oh, I meant class D, not G or H... I think.

I think Class H is that switched rail thing, like Carver?
And D is switchmode.

That leaves G... hmmmm...

Guess I have to get a newer text book, eh?

Maybe it's time to get those straight in my mind.

Oh yeah - more on "linearity"

If you look at the transfer curve of any device, they tend to be most linear (straight line) in the middle of the curve. That means that for a given input increase, the output changes the same amount for each increase/decrease.
(ie. "linear" - a 1:1 relationship being maintained)

At the ends of the curves (all the way on, and all the way off) the relationship is not so good. In fact the transfer curve looks like an "S."

What this means is that when almost off - at the bottom of the curve the device is not very linear - you need much more drive to get not so much output, UNTIL you get past the curve of the "S" and it gets linear.

So, two things are important here: 1) for class AB and B circuits, you're almost off (near "cut-off") so the curve to begin with is not linear. 2) Class A is in the middle of the most linear portion of the curve.

In class AB, the idea is usually to bias so that you start out just above the non-linear portion, so that all positive going drive sends you only up through the linear portion of the curve. Of course, when any one side of a Push-Pull pair is driven toward "off" it travels back down the non-linear area. Thus the gains do not sum perfectly, and the earlier comment about "crossover distortion" applies.

In class A, you can drive out of the linear region on peaks! This is *part* of the sound you get from pure triode Class A ZERO feedback amps. There is effectively some *compression* when driven hard into the ends of the linear region (more signal does not get you equally more output). Which in part accounts for why small pure triode amps can sound like they play louder than very linear solid state amps.

Feedback changes this, since it forces the input to be whatever is needed to keep things linear (in this example, more drive is required until a 1:1 gain relationship is acheived).

Here when I say "1:1" gain relationship it does not mean that the gain is unity, but if it is 1 input unit then you get out "N" output units, 5 input: 5N output.

Which is best? That's entirely unclear.

To add to the design mix - not all devices, tubes or transistors *ARE LINEAR* by themselves! Most are NOT. Most
are either entirely non linear everywhere or linear over only a small portion of their range. This is a large part of why feedback of all sorts is employed in practice. Keep in mind that the main reason a cathode follower is linear, is that it uses 100% feedback! :- )